A typical cellular radio system comprises a fixed base station network, and a subscriber terminal communicates with one or more base stations of the network. A base station forwards the communication that arrives from a subscriber terminal. While moving or remaining stationary, subscriber terminals may transmit messages via the base stations both to each other and to subscriber terminals of other telephone systems. The transmission of messages is possible when the subscriber terminals are located in the coverage area of the base station network. In order for a subscriber terminal to be able to use the services provided by the cellular radio system, it should maintain a connection to at least one base station under all circumstances. When a subscriber terminal does not use the services provided by the base station network, it does not need a connection to the base station network but it listens to the base stations in an idle mode. When the subscriber terminal moves in the base station network from the coverage area of one base station to the coverage area of another base station, this creates a need to change the channel or the base station.
In a typical cellular radio system, a subscriber terminal communicates only with one base station at a time, even though especially for example in a CDMA system the subscriber terminal may also communicate simultaneously with several base stations. In a prior art soft hand-off, the connection to the base station network is maintained despite the hand-off. In such a hand-off, the base station is usually changed. The prior art also comprises a softer hand-off where the base station is not changed, but the sector of the base station used is changed. A soft and a softer hand-off are called make-before-break-type hand-offs, which means that a new connection is set up for the subscriber terminal before the connection to the previous base station is terminated. The frequency band used is not changed in either hand-off.
A cellular radio system usually comprises a TRAU (Transcoder/Rate Adaptor Unit) that is placed for example in connection with a base station controller or a mobile services switching centre. The TRAU source-encodes the signal and adapts the signal transmission rate to the transmission network, for example a Public Switched Telephone Network (PSTN). The TRAU forms TRAU frames that it transmits to the base station. The TRAU acts as a speech coder, i.e. a vocoder. The coding of the signal reduces the signal data rate for example in a transmission line to the base station. The vocoder and the base station transmit to and receive from each other data packets that form the aforementioned TRAU frames.
In a soft hand-off, a subscriber terminal communicates with several base stations simultaneously. In a soft hand-off, the subscriber terminal transmits to the base station a signal comprising the same information, the signal being forwarded to the vocoder. Also, the subscriber terminal receives a signal comprising the same information from the base station, which has received this signal from the vocoder. In cellular radio systems, the signal routing from the vocoder to the different base stations varies considerably, and the length of the transmission path between the vocoder and a base station may be great. Especially the length of the transmission path causes a delay in the signal to be transmitted between the vocoder and a base station. The length of the delay varies largely depending especially on the signal routing used. The delay causes problems during a soft hand-off in the simultaneous transmission of a signal by the base stations to a subscriber terminal. The prior arrangements typically determine the delay differences of the signals received by the vocoder. However, the determination of the delay differences requires continuous calculation.